Absolute Temperature — Definition and Kelvin Scale

In physics, Absolute Temperature is defined as a temperature scale whose starting point (zero) is absolute zero.

Scientifically, absolute temperature is directly proportional to the average translational kinetic energy of the ideal gas molecules in a system. When absolute temperature doubles, the average kinetic energy of the particles doubles.

• It is a fundamental thermodynamic parameter.
• It can never be negative. The lowest possible value is exactly 0.

Absolute Zero (0 Kelvin) is the theoretical temperature at which all thermal motion of atoms and molecules completely stops. It is the state of lowest possible energy.

In standard scales:

• 0 Kelvin (0 K)
• −273.15 °Celsius
• −459.67 °Fahrenheit

According to the Third Law of Thermodynamics, it is impossible for any system to actually reach absolute zero, though scientists have cooled matter to within billionths of a degree of it.

The Kelvin scale is the SI unit for temperature. It was proposed by Lord Kelvin in 1848.

Unlike Celsius and Fahrenheit, Kelvin does not use the degree symbol (°). We say '300 Kelvin' or '300 K', not '300 degrees Kelvin'.

Conversion Formula: T(Kelvin) = T(Celsius) + 273.15

For most school calculations, we round 273.15 to 273:

• Boiling point of water: 100°C = 373 K
• Freezing point of water: 0°C = 273 K
• Normal human body temperature: 37°C = 310 K

Absolute temperature must be used in all fundamental physics and chemistry equations, particularly the Ideal Gas Law: PV = nRT (where T must be in Kelvin).

If you use Celsius, calculations will break down (especially at 0°C, which would imply zero volume or pressure, which is physically impossible).